/en/current/index.html
Highlights and News at MBI
Earlier Highlights are found in the archive
19th January 2016: Invisible light flash ignites nano-fireworks
A team of researchers from the Max Born Institute in Berlin and the University of Rostock demonstrated a new way to turn initially transparent nanoparticles suddenly into strong absorbers for intense laser light and let them explode. ...more.
 
22nd December 2015: Isolating Water's Impact on Vibrations within DNA
Researchers in Berlin have found that water at the interface with DNA contributes prominently to subpicosecond structure fluctuations, and leaves hydrogen bonds between DNA and water intact ...more.
 
9th December 2015: On the sad occasion of the death of Prof. Dr. Wolfgang Sandner
The Forschungsverbund Berlin e.V. (FVB) mourns the loss of laser physicist Prof. Dr. Wolfgang Sandner. On December 5th, he died unexpectedly at the age of 66. Wolfgang Sandner shaped the development of the Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) where he was one of the three directors from 1993 until 2013. ... more.
 
1st November 2015: On November 1st, Prof. Stefan Eisebitt has been appointed as W3-S-Professor in Laser Physics at the TU Berlin in combination with the position of a director at the Max-Born-Institute
Stefan Eisebitt studied physics in Cologne and carried out his research towards a M.Sc. thesis at the Institut für Festkörperforschung (IFF) of the Forschungszentrum Jülich. For his Ph.D. work, Eisebitt stayed for three years at the University of British Columbia, Canada, on a DAAD fellowship. His thesis, submitted 1996 to Cologne University, was focused on the interplay of geometric and electronic structure in nanoscale materials, employing XUV and soft x-ray spectroscopy. ... more.
 
15th October 2015: Energy exchange in highly ionized nanoparticles
Excited atoms often decay via the emission of radiation, a process that is known as fluorescence. A different scenario can emerge when an excited atom is surrounded by other excited atoms, ions and electrons. Such a situation is achieved when an intense laser pulse interacts with a nanoscale object. In this case, an excited atom can decay by transferring its excess energy to another particle in the environment ... more.
 
28th September 2015: Hot means slow: Electron plasma oscillations tuned down with light
Oscillations of an optically heated electron plasma depend sensitively on the plasma temperature. Ultrafast heating and cooling of a plasma in the semiconductor zinc oxide (ZnO) leads to pronounced shifts of plasma frequency, holding a strong potential for novel switching applications in optoelectronics. ... more.
 
18th August 2015: How to Flow Ultrathin Water Layers - A Liquid Flatjet for X-Ray Spectroscopy
A major advance in solution phase soft-x-ray spectroscopy has been achieved utilizing a new liquid flatjet system, paving the way for novel steady-state and time-resolved experiments ... more.
 
13th August 2015: A new approach towards solving mysteries of the interstellar medium
It is one of the most intriguing questions in astrochemistry: the mystery of the diffuse interstellar bands (DIBs), a collection of about 400 absorption bands that show up in spectra of light that reaches the earth after having traversed the interstellar medium. Despite intense research efforts over the last few decades, an assignment of the DIBs has remained elusive, although indications exist that they may arise from the presence of large hydrocarbon molecules in interstellar space. Recent experiments at the Max Born Institute lend novel credibility to this hypothesis. ... more.
 
5th August 2015: European researchers light the way towards top-level laser science and innovations
LASERLAB-EUROPE, the consortium of major European laser research organisations, enters a new phase of collaboration from 2015 until 2019. In a very competitive call the consortium has been successful in securing EC funding of 10 million euros in Horizon 2020. ... more.
 
29th June 2015: Probing molecular chirality on a sub-femtosecond timescale
New nonlinear all-optical method of detecting chiral molecules is proposed and demonstrated. It is a lot more sensitive than standard all-optical techniques and also allows for sub-femtosecond temporal resolution of chiral dynamics. ... more.
 
28th May 2015: Are rogue waves predictable?
A comparative analysis of rogue waves in different physical systems comes to the surprising conclusion that these rare events are not completely unpredictable. ... more.
 
25th May 2015: How long does it take an electron to tunnel?
The combination of ab-initio numerical experiments and theory shows that optical tunnelling of an electron from an atom can occur instantaneously. ... more.
 
10th April 2015: Rubicon fellowship for Geert Reitsma
The Netherlands Organisation for Scientific Research (NWO) rewards MBI researcher Geert Reitsma a prestigious Rubicon Grant for his proposal "Filming biomolecules in action". With this grant he will continue his career at the MBI.. ... more.
 
26th March 2015: Fast Gold
A new laser plasma acceleration scheme for heavy ions is found, leading to a significant boost of their
kinetic energy by Coulomb explosion. ... more.
 
24th March 2015: The exciting story of He atoms in strong laser fields
Astonishingly, the interaction of atoms with intense laser radiation does not necessarily lead to ionization but also to bound excited states. As investigated in a recent publication, H. Zimmermann and coworkers have experimentally confirmed predictions of a model that provides an intuitive understanding of why and how strong-field excitation happens. ... more.
 
24th March 2015: Classical or not? Physics of nanoplasmas
The interaction of an intense laser pulse with a nanometer-scale particle results in the generation of an expanding nanoplasma. In the past, nanoplasma dynamics were typically described by classical phenomena, like the thermal emission of electrons. In contrast, a new study on the interaction of intense near-infrared (NIR) laser pulses with molecular oxygen clusters now demonstrates that phenomena, which can only be described quantummechanically, play an important role. ... more.
 
11th February 2015: New Insights into the photophysics of the DNA base thymin
DNA stores our genetic code. Solar UV radiation has sufficiently high energy to basically break bonds of the DNA and thus cause DNA damage. Although DNA (e. g. in our skin cells) is exposed to intense UV light irradiation, DNA proves to be surprising photostable. ... more.
 
10th February 2015: Nonlinear resonance disaster in the light of ultrashort pulses
Ultrashort light pulses from modern lasers enable temporal resolution of even the fastest processes in molecules or solid-state materials. ... more.
 

 
More detailed Information:

 


Invisible light flash ignites nano-fireworks

19th January 2016

A team of researchers from the Max Born Institute in Berlin and the University of Rostock demonstrated a new way to turn initially transparent nanoparticles suddenly into strong absorbers for intense laser light and let them explode.

Intense laser pulses can transform transparent material into a plasma that captures energy of the incoming light very efficiently. Scientists from Berlin and Rostock discovered a trick to start and control this process in a way that is so efficient that it could advance methods in nanofabrication and medicine. The light-matter encounter was studied by a team of physicists from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Berlin and from the Institute of Physics of the University of Rostock.

The researchers studied the interaction of intense near-infrared (NIR) laser pulses with tiny, nanometer-sized particles that contain only a few thousand Argon atoms - so-called atomic nanoclusters. The visible NIR light pulse alone can only generate a plasma if its electromagnetic waves are so strong that they rip individual atoms apart into electrons and ions. The scientists could outsmart this so-called ignition threshold by illuminating the clusters with an additional weak extreme-ultraviolet (XUV) laser pulse that is invisible to the human eye and lasts only a few femtoseconds (a femtosecond is a millionth of a billionth of a second). With this trick the researchers could "switch on" the energy transfer from the near-infrared light to the particle at unexpectedly low NIR intensities and created nano-fireworks, during which electrons, ions and colourful fluorescence light are sent out from the clusters in different directions (Figure 1). Their results open unprecedented opportunities for both fundamental laser-matter research and applications and was published in the latest issue of Physical Review Letters.

The experiments were carried out at the Max Born Institute at a 12 meter long high-harmonic generation (HHG) beamline. "The observation that argon clusters were strongly ionized even at moderate NIR laser intensities was very surprising", explains Dr. Bernd Schütte from MBI, who conceived and performed the experiments. "Even though the additional XUV laser pulse is weak, its presence is crucial: without the XUV ignition pulse, the nanoparticles remained unaffected and transparent for the NIR light (Figure 2)." Theorists around Prof. Thomas Fennel from the University of Rostock modelled the light-matter processes with numerical simulations and uncovered the origin of the observed synergy of the two laser pulses. They found that only a few seed electrons created by the ionizing radiation of the XUV pulse are sufficient to start a process similar to a snow avalanche in the mountains. The seed electrons are heated in the NIR laser light and kick out even more electrons. "In this avalanching process, the number of free electrons in the nanoparticle increases exponentially", explains Prof. Fennel. "Eventually, the nanoscale plasma in the particles can be heated so strongly that highly charged ions are created."

The novel concept of starting ionization avalanching with XUV light makes it possible to spatially and temporally control the strong-field ionization of nanoparticles and solids. Using HHG pulses paves the way for monitoring and controlling the ionization of nanoparticles on attosecond time scales, which is incredibly fast. One attosecond compares to a second as one second to the age of the universe. Moreover, the ignition method is expected to be applicable also to dielectric solids. This makes the concept very interesting for applications, in which intense laser pulses are used for the fabrication of nanostructures. By applying XUV pulses, a smaller focus size and therefore a higher precision could be achieved. At the same time, the overall efficiency can be improved, as NIR pulses with a much lower intensity compared to current methods could be used. In this way, novel nanolithography and nanosurgery applications may become possible in the future.

Original Publication: Physical Review Letters 116, 033001
Ionization avalanching in clusters ignited by extreme-ultraviolet driven seed electrons

Full Citation:
Bernd Schütte, Mathias Arbeiter, Alexandre Mermillod-Blondin, Marc J. J. Vrakking, Arnaud Rouzée, Thomas Fennel
"Ionization Avalanching in Clusters Ignited by Extreme-Ultraviolet Driven Seed Electrons"

DOI: 10.1103/PhysRevLett.116.033001

Contact

Dr. Bernd Schütte

Schuette

Fig. 1: Nano-fireworks in an argon nanoparticle are ignited by a moderately intense and invisible XUV laser pulse. A subsequent visible laser pulse heats the nanoparticle very efficiently, resulting in its explosion. Electrons and ions move in different directions and send out fluorescence light in various colors. Without the XUV pulse the nanoparticle would remain intact.

 

Abb. 1 (click to enlarge)  
Schuette

Fig. 2: Ion charge spectra measured from argon nanoparticles. Using an XUV ignition pulse, only a few singly-charged ions are observed (black spectrum). By adding an NIR heating pulse, highly charged ions up to Ar8+ are generated (red spectrum).

Fig. 2 (click to enlarge)  
 
     
 


Isolating Water's Impact on Vibrations within DNA

22nd December 2015

The work on DNA has been highlighted in a press release by the American Institute of Physics (AIP). Link (PDF-File)

DNA-Helix Schematic structure of a DNA helix and spatial distribution of water molecules.

Publications:

B. Guchhait, Y. Liu, T. Siebert, T. Elsaesser, Ultrafast vibrational dynamics of the DNA backbone at different hydration levels mapped by two-dimensional infrared spectroscopy, Structural Dynamics 3, 043202/1-15 (2016)

T. Siebert, B. Guchhait, Y. Liu, R. Costard, T. Elsaesser, Anharmonic backbone vibrations in ultrafast processes at the DNA-water interface, J. Phys. Chem. B 119, 9670-9677 (2015)

Contact

Dr. Torsten Siebert, phone +49 30 63921414
Prof. Thomas Elsaesser, phone +49 30 63921400

 
     
 


On the sad occasion of the death of Prof. Dr. Wolfgang Sandner

9th December 2015

WSandner

The Forschungsverbund Berlin e.V. (FVB) mourns the loss of laser physicist Prof. Dr. Wolfgang Sandner. On December 5th, he died unexpectedly at the age of 66. Wolfgang Sandner shaped the development of the Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) where he was one of the three directors from 1993 until 2013.

Wolfgang Sandner - Photo: Ralf Günther  

For Marc Vrakking, Chair of the FVB's board of directors and director at the MBI, an outstanding scientist who created significant opportunities for European laser physics in the political arena, has passed away far too soon. "Wolfgang Sandner strongly believed that the necessary facilities of an Extreme Light Infrastructure (ELI) could only be established through a common European effort", said Vrakking. "His death came absolutely unexpected - I met with him only two weeks ago, when he talked to me enthusiastically about his work and plans as the Director General of ELI." As recently as the middle of October, Sandner could witness the inauguration of the ELI-Beamlines facility in Dolní Břežany, in the Czech Republic.

Wolfgang Sandner studied physics at the University of Freiburg, where he graduated with a PhD in atomic physics in 1979. Soon he began to focus on laser physics. After he held chairs at the universities of Wuerzburg, Freiburg and Knoxville (Tennessee) he joined the MBI in 1993 and obtained a chair in physics at the Technische Universität Berlin. During his time at the MBI, Prof. Sandner performed pioneering experiments on the quantum three-body Coulomb problem in highly excited atoms and he investigated laser-matter interactions in strong laser fields. In addition, he worked on the development of ultrashort and high intensity UV- and x-ray lasers.

From 2003 to 2013, he coordinated the integrated infrastructure initiative Laserlab Europe, a collaboration of Europe's 30 largest laser research facilities, and as such promoted European laser physics. From 2010 to 2012, Sandner was the President of the Deutsche Physikalische Gesellschaft (DPG). Since 2013, he coordinated the development of the Extreme Light Infrastructure (ELI) as Director General of the ELI-Delivery Consortium. In planned ELI facilities in Rumania, Hungary, and the Czech Republic, as well as at a further location not yet named, the highest intensity lasers of the world will be used.

 
     
 


On November 1st, Prof. Stefan Eisebitt has been appointed as W3-S-Professor in Laser Physics at the TU Berlin in combination with the position of a director at the Max-Born-Institute.

1st November 2015

On November 1st, Prof. Stefan Eisebitt has been appointed as W3-S-Professor in Laser Physics at the TU Berlin in combination with the position of a director at the Max-Born-Institute. Stefan Eisebitt studied physics in Cologne and carried out his research towards a M.Sc. thesis at the Institut für Festkörperforschung (IFF) of the Forschungszentrum Jülich. For his Ph.D. work, Eisebitt stayed for three years at the University of British Columbia, Canada, on a DAAD fellowship. His thesis, submitted 1996 to Cologne University, was focused on the interplay of geometric and electronic structure in nanoscale materials, employing XUV and soft x-ray spectroscopy. At the IFF and the Stanford Linear Accelerator Center (SLAC), he worked on methods to exploit the coherence properties of x-rays in order to investigate the (dynamic) geometric structure on the nanoscale. Ultimately, these efforts led to a high resolution x-ray holography method, which allows temporally and spatially resolved measurements of e.g. spin structures. In early 2002 Eisebitt moved to Berlin, where he headed one of the first in-house research groups at BESSY. In 2008, he accepted a full professorship at the Technische Universität Berlin, where he established the research group "Nanometer Optics and X-ray Scattering". From then on, the center of his attention moved to the field of "Femtomagnetism", the study of ultrafast dynamics in magnetic systems. In suitable materials, laser pulses can be used to quench or alternatively to switch magnetization on an ultrafast time scale. While this is of interest for data storage applications, Eisebitt's main interest is to understand the underlying fundamental processes between electrons, spins and phonons, which in many aspects are still unknown. The most important tools for such studies are femtosecond laser pulses over the entire spectral range from terahertz to x-rays - hence synchrotron radiation from storage rings or free-electron lasers are used in addition to laser-based lab sources for experiments. As the successor of Wolfgang Sandner, Stefan Eisebitt will be responsible for Division B at the Max-Born-Institute. On behalf of all MBI employees we congratulate Stefan Eisebitt on his appointment and we are looking forward to working together with you.

Contact

Prof. Dr. Stefan Eisebitt

Tel. 030 6392 1300

 
     
 


Energy exchange in highly ionized nanoparticles

15 October 2015

Excited atoms often decay via the emission of radiation, a process that is known as fluorescence. A different scenario can emerge when an excited atom is surrounded by other excited atoms, ions and electrons. Such a situation is achieved when an intense laser pulse interacts with a nanoscale object. In this case, an excited atom can decay by transferring its excess energy to another particle in the environment. Researchers from the Max-Born-Institut in Berlin, the University of Rostock, and the University of Heidelberg found evidence for such an energy exchange involving electrons that are trapped within a nanocluster. They observed a so far unidentified peak in the electron spectrum following the ionization of a nanocluster by a near-infrared (NIR) laser pulse. The researchers attributed this signal to the relaxation of one electron from an excited Rydberg atom and the simultaneous transfer of the excess energy to a second electron that can escape from the cluster. The obtained results, which were published in Nature Communications, are of universal nature and expected to play an important role in other nanoscale systems including biomolecules.

Interatomic Coulombic decay (ICD) describes the relaxation of an excited atom by transferring its excess energy to a neighboring atom that gets ionized. This effect has received significant attention in recent years, as it may be a source of radiation damage in biological systems. At the same time, it was proposed to exploit ICD for novel cancer therapies. So far, ICD has been observed following the ionization or excitation of clusters by high-energy photons in the extreme-ultraviolet (XUV) and X-ray range. In contrast, it had not been expected that ICD could be induced by low-energy photons in the NIR regime.

The ionizaton of a cluster by an intense NIR laser pulse triggers highly complex dynamics. A so called nanoplasma is formed that consists of a large number of ions and electrons interacting with each other. Recombination of electrons and ions has been found to result in the generation of Rydberg atoms and ions, which can decay via fluorescence. However, in a strongly ionized cluster, Rydberg atoms may also relax via correlated electronic decay (CED) processes similar to ICD, i.e. without the emission of radiation. In CED, one electron can relax from a Rydberg state to the ground state and transfer its excess energy to a second electron, which is either located in the same atom, in the nanoplasma, or which is in a Rydberg state of a nearby atom (see Figure 1). Using this additional energy, the second electron can escape from the cluster. "Even though CED may be expected in nanoplasmas, the effect had neither been observed in experiments nor had it been predicted by theoretical models.", explains Dr. Bernd Schütte from the Max-Born-Institut. "The major challenge in the experiment was to find suitable conditions that allow a direct observation of correlated electronic decay."

Just recently, the researchers were rewarded for their search and found evidence of CED in the electron spectrum from argon clusters ionized by an intense NIR laser pulse. Their results have now been published in Nature Communications. The emergence of a peak in the energy spectrum of emitted electrons that is close in energy to the atomic ionization potential (see Figure 2) was found to be the signature of an electronic decay process involving bound atomic states. Surprisingly, the scientists found that the energy exchange between electrons takes place almost 100 picoseconds after the cluster is ionized. This is much slower than for typical ICD processes that proceed on 100 femtoseconds timescales.

Support for this explanation was obtained by modeling the complex dynamics taking place in the expanding clusters by the group of Prof. Thomas Fennel from the University of Rostock. "The tricky aspect of the experiment is that the charged and expanding cluster disturbs the electrons emitted via CED. Electrons that have been emitted in early expansion stages will have lost their specific bound-state signatures.", explains Fennel. The ICD expert Dr. Alexander Kuleff from the University of Heidelberg adds "Our calculations show that ICD between lowly excited argon atoms takes place on a timescale of 200 femtoseconds, but the process significantly slows down, when higher Rydberg states are involved. This is in good agreement with the experiment, which suggests that the observed electrons are emitted from higher Rydberg orbitals."

Although the first experiments on clusters with intense NIR laser pulses were already performed in the 1990s, it took a long time to observe correlated electronic decay in expanding nanoplasmas for the first time. One reason why this effect could not be revealed in previous experiments is that it can only be directly observed in a very small range of laser intensities and cluster sizes. However, after having understood the involved dynamics, the researchers could show that CED has a universal nature. The process was observed in all the investigated clusters, which include atomic krypton and xenon clusters as well as molecular oxygen clusters. "CED takes place as soon as a nanoplasma is born within the cluster and excited states are populated by recombination", explains Dr. Arnaud Rouzée from the Max-Born-Institut, adding "CED is therefore expected to be important also for experiments, in which intense XUV and X-ray laser pulses that interact with nanoscale objects, including biomolecules." Further experiments are under way in order to elucidate the overall significance of correlated electronic decay in highly excited complex systems.

Original Publication: Nature Communications 6, DOI:10.1038/ncomms9596
"Observation of correlated electronic decay in expanding clusters triggered by near-infrared fields"

Full Citation:
Bernd Schütte, Mathias Arbeiter, Thomas Fennel, Ghazal Jabbari, Alexander I. Kuleff, Marc J. J. Vrakking and Arnaud Rouzée, "Observation of correlated electronic decay in expanding clusters triggered by near-infrared fields", Nature Communications 6, 8596 (2015)

Contact

Dr. Bernd Schütte

Prof. Marc J. J. Vrakking

Dr. Arnaud Rouzée

figure follows

Fig. 1: Correlated electronic decay in clusters: An electron in a Rydberg state can relax to the ground state and transfer its excess energy (a) to a second electron that occupies a Rydberg state in the same atom, (b) to a quasifree electron in the environment, or (c) to an electron that occupies a Rydberg state in a second atom.

 

Fig. 1 (click to enlarge)  
figure 2 follows

Fig. 2: Electron spectrum measured after the ionization of argon clusters by intense NIR pulses. The gray area represents thermal electron emission. In addition, a peak structure (blue area) with a prominent peak close to the ionization potential of atomic argon (15.76 eV) appears. This structure can be explained by correlated electronic decay.

Fig. 2 (click to enlarge)  
 
     
 


Hot means slow: Electron plasma oscillations tuned down with light

28th September 2015

Oscillations of an optically heated electron plasma depend sensitively on the plasma temperature. Ultrafast heating and cooling of a plasma in the semiconductor zinc oxide (ZnO) leads to pronounced shifts of plasma frequency, holding a strong potential for novel switching applications in optoelectronics.

A plasma is a special state of matter in which a large number of electrons form are negatively charged cloud of particles which is separated from a positively charged background of ions. Plasma exists in many systems including hot stars, the ionosphere and other ionized gases, as well as solid state materials. The electric forces between electrons and ions allow for generating periodic spatial motions of the electron cloud relative to the ions, so called plasma oscillations or plasmons. Recently, plasmons in metals and semiconductors have received strong interest. They display peculiar optical properties and hold strong potential for applications in high-speed optoelectronics and optical microscopy with sub-wavelength spatial resolution.

A basic and interesting question is: Can one manipulate plasma oscillations with light and, in particular, modify their frequency? This would allow for switching the electric and optical properties for a short period of time, changes most helpful for novel optoelectronic devices. In the current issue of Physical Review Letters [115, 147401 (2015)], a joint research team from the Max-Born-Institute and Humboldt University in Berlin demonstrates a novel concept for ultrafast plasmon switching in the semiconductor ZnO (Movie). In their experiments, the researchers investigated plasma oscillations in a 100 nanometer thick crystalline ZnO layer containing a high density of approximately 1020 free electrons per cubic centimeter. Plasma oscillations are excited by an infrared pulse of 150 fs duration (1 fs = 10-15 s) and the frequency shift of the infrared plasmon absorption band is measured with a second delayed and much weaker probe pulse. The shift of the absorption band allows for extracting the momentary plasma frequency as a function of time (Fig. 1). The experiments give direct evidence of a transient shift of plasma oscillations to lower frequency. The strong frequency reduction by 20% lasts for only 400 fs after which the original plasma frequency is restored. Over the period of the experiment, the electron density remains unchanged.

The physical origin of the frequency reduction lies in the transient heating of the electron plasma by the infrared excitation pulse. The electrons reach a peak temperature of ≈3300 K and populate a very wide range of the conduction band of ZnO (Fig. 2). In this range, the average electron mass is higher than in the initial state and, thus, the plasma frequency is reduced. The hot electrons transfer most of their thermal excess energy to the crystal lattice within some 400 fs. As a result, both the average electron mass and the plasma frequency return to their original values. All experimental observations are in excellent agreement with theoretical calculations.

TybPressAbb2 Fig. 1: Experimentally observed time-dependent shift of the plasma frequency in a thin ZnO layer. Left: 3D-plot of the absorption change as a function of the probe frequency and time delay between pump and probe pulses. Right: concept of a transient difference spectrum. The cold plasma (blue) shows an absorption peak at the plasma frequency of the cold electron gas. The pump pulse heats the plasma resulting in a red-shift of the plasmon resonance (red). In the time-resolved experiments we measured the so called difference spectrum, i.e., the absorption of the hot plasma minus that of the cold plasma (black).
TybPressAbb1 Fig. 2: The conduction band of ZnO shows a non-parabolic band structure, i.e., the electron energy as a function of the electron momentum follows a hyperbola rather than a parabola. As a result electrons at the conduction band minimum are quite light (low energy, small mass) compared to the much heavier electrons (large mass) at high energies. A cold plasma (left) contains essentially light electrons whereas a hot plasma (right) contains many heavy electrons at high energies.
Movie Animation: Right: plasma oscillations in a thin ZnO layer. Negatively charged electrons (blue clouds) oscillate collectively versus the positively charged ions (red dots). Left: Such a plasma oscillation resembles strongly a classical pendulum, i.e., a massive ball hanging on a elastic spring. (i) For negative times t < 0 (time counter upper left), the oscillation frequency is quite high due to the small mass of the electrons. (ii) During the period 0 < t < 100 fs fs the pump pulse heats the electron plasma (lighters below, temperature display upper right) resulting in an elevated mass of the electrons in ZnO (right) or increased mass in the pendulum (left). (iii) For t >100 fs the probe pulse measures the plasma oscillation frequency again, now showing a distinctly slower motion.

Original Publication: Physical Review Letters 115, 147401
Ultrafast Nonlinear Response of Bulk Plasmons in Highly Doped ZnO Layers

Tobias Tyborski, Sascha Kalusniak, Sergey Sadofev, Fritz Henneberger, Michael Woerner, and Thomas Elsaesser

Contact

Dr. Michael Woerner Tel. 030 6392 1470
Prof. Dr. Thomas Elsaesser Tel. 030 6392 1400

 
     
 


How to Flow Ultrathin Water Layers - A Liquid Flatjet for X-Ray Spectroscopy

18 August 2015

A major advance in solution phase soft-x-ray spectroscopy has been achieved utilizing a new liquid flatjet system, paving the way for novel steady-state and time-resolved experiments.

Element-specific x-ray methods play a key role in determining the atomic structure and composition of matter and functional materials. X-ray spectroscopy is sensitive to the oxidation state, the distances, coordination number and species of the atoms immediately surrounding the selected element. A large variety of x-ray spectroscopic techniques have been applied to gas-phase, bulk liquid or solid-state samples, or have been used to probe molecular systems at interfaces. X-ray spectroscopy is predominantly done at large-scale synchrotron facilities, or in more recent years with x-ray free electron lasers, probing steady-state and time-resolved material properties.

Solution phase soft-x-ray absorption spectroscopy (XAS, energy range approximately from 0.2 - 1.5 keV) is not an easy method: experiments need to be done under vacuum conditions, an environment obviously incompatible with the high vapor pressure of water. Furthermore, if measured in transmission, absorption cross sections demand sample thicknesses in the micrometer and submicrometer range (1 micrometer = 10-6m = one millionth of a meter). Alternatively, if secondary signals such as x-ray fluorescence are measured, the experiment is limited to comparably large solute concentrations. Using sample cells with thin membrane windows enables control of appropriate sample thicknesses, but sample degradation upon x-ray illumination (or upon pump laser illumination in time-resolved experiments) makes this approach disadvantageous for photolabile molecular systems. Sample refreshment is possible with a liquid jet, generated by pumping a solution through a nozzle with a small orifice, into the vacuum chamber. Single liquid jets have, however, difficulties to implement the required (sub)micron thicknesses.

A collaboration between scientists from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI), the Helmholtz-Zentrum Berlin (HZB) and the Max Planck Institute for Dynamics and Self-Organization (MPIDS) have now demonstrated the successful implementation of a liquid flatjet with a thickness in the μm range, allowing for XAS transmission measurements in the soft-x-ray regime. Here a phenomenon well known in the field of fluid dynamics has been applied: by obliquely colliding two identical laminar jets, the liquid expands radially, generating a sheet in the form of a leaf, bounded by a thicker rim, orthogonal to the plane of the impinging jets.

The novel aspect here is that a liquid water flatjet has been demonstrated with thicknesses in the few micrometer range, stable for tens to hundreds of minutes, fully operational under vacuum conditions (‹10-3mbar). For the first time, soft x-ray absorption spectra of a liquid sample could be measured in transmission without any membrane. The x-ray measurements were performed at the soft x-ray synchrotron facility BESSYII of the Helmholtz-Zentrum Berlin. This technological breakthrough opens up new frontiers in steady-state and time-resolved soft-x-ray spectroscopy of solution phase systems.

LiquidFlatjetTOC Fig.: Liquid flatjet system, showing the two nozzles from which two impinging single jets form a 1 mm wide and 5 mm long liquid water sheet with a thickness of 1 - 2 μm as determined by measuring the transmission at the oxygen K absorption edge (left), with which XAS measurements in transmission can be made on aqueous solutions, as exemplified with the nitrogen K absorption edge spectrum of ammoniumchloride (right).
Abb. (click to enlarge)  

Original publication: Structural Dynamics 2, 054301 (2015)
A liquid flatjet system for solution phase soft-x-ray spectroscopy

Maria Ekimova, Wilson Quevedo, Manfred Faubel, Philippe Wernet, Erik T.J. Nibbering

Contact

Dr. M. Ekimova
Dr. Erik T.J. Nibbering

 
     
 


A new approach towards solving mysteries of the interstellar medium

13 August 2015

It is one of the most intriguing questions in astrochemistry: the mystery of the diffuse interstellar bands (DIBs), a collection of about 400 absorption bands that show up in spectra of light that reaches the earth after having traversed the interstellar medium. Despite intense research efforts over the last few decades, an assignment of the DIBs has remained elusive, although indications exist that they may arise from the presence of large hydrocarbon molecules in interstellar space. Recent experiments at the Max Born Institute lend novel credibility to this hypothesis.

Among the hydrocarbons that are possible carriers of the DIBs, polycyclic aromatic hydrocarbons (PAHs) are considered to be particularly promising. The presence of PAH molecules was previously inferred in many astronomical objects, as well as in the interstellar medium of the Milky Way. However, within the astronomical community, the linewidths of the DIBs, which are indicative of the lifetimes of the excited states that are involved in the absorption process, are often considered as an argument that speaks against the PAHs. The new experiment was performed in collaboration with scientists from the university of Lyon and aided by theoretical input from scientists at the universities of Heidelberg, Hyderabad and Leiden. It has been shown that the lifetimes of excited states of small to medium-size PAHs are consistent with the linewidths that are observed for the DIBs.

In the experiments, a series of small to medium-size PAH molecules (naphthalene, anthracene, pyrene and tetracene, containing 2-4 benzene-like aromatic rings), were ionized by an ultrashort extreme-ultraviolet (XUV) laser pulse. As a result of electron correlation, the absorption of an XUV photon not only led to removal of one of the electrons, but furthermore to electronic excitation of the molecular ion left behind. The lifetimes of these excited cationic electronic states were monitored by probing the ions with a moderately strong, time-delayed infrared (IR) laser pulse. When the ions are formed, the electronic excitation is at its highest, and only one or a few IR photons are needed to remove a second electron. However, a little later, when the ion relaxes and energy is transferred from the electronic to the vibrational degrees of freedom, more IR photons are needed to remove the second electron. In other words, monitoring the formation of doubly-charged ions as a function of the time delay between the XUV and IR laser pulses allowed extraction of the lifetimes of the states formed by the XUV ionization process. As it turned out, and as was further supported by high-level calculations, these lifetimes of a few 10s of femtoseconds are well within the range of what is required for potential carriers of the DIBs.

Beyond the implications for the DIBs, the new experiments have implications for the further development of attosecond science. One of the most sought-after goals in attosecond science at the moment, is the observation of charge migration, i.e. ultrafast (attosecond to few-femtosecond) motion of an electron or hole through a molecular structure. It has been proposed that charge migration may provide new opportunities for control of chemical reactivity, a goal that is as old as the chemical research itself. First indications that attosecond to few-femtosecond time-scale dynamics can be observed in polyatomic molecules were obtained by researchers at the university of Milano last year. The PAH molecules that were investigated in the experiments at MBI represent the largest molecular species yet to which ultrafast XUV-IR pump-probe spectroscopy has been applied. Besides the insights into ultrafast electronic relaxation obtained from the current work, the theoretical work performed in order to interpret the experiments suggests that PAH molecules are also ideal candidates for observing attosecond to few-femtosecond timescale charge migration. Such experiments will therefore be attempted next.

UXVncomms Abb.: Schematic of the experiment. (a) Schematic of the XUV-induced dynamics in PAH molecules studied in this paper. Excited states are created in the valence shell of the cation through one of two possibilities, namely the formation of a single-hole configuration or the formation of a 2hole-1particle configuration (involving a shake-up process) (left) (IP stands for Ionization potential). The cation can be further ionized by the IR probe laser, provided that non-adiabatic relaxation has not taken place yet (middle). After relaxation, the IR probe cannot ionize the cation anymore (right). (b) Two-colour XUV-IR ion signals measured in the case of anthracene, as a function of the detected mass-to-charge ratio and the XUV-IR delay. XUV-only and IR-only signals have been subtracted. The XUV pump and IR probe pulses overlap at zero delay (black dashed line). A red colour corresponds to a signal increase, while a blue colour signifies depletion. For positive XUV-IR delays, a very fast dynamics is observed for the doubly charged anthracene ion (A2+, m/q=89). As explained in the text, the measurement reflects non-adiabatic relaxation in the anthracene cation (A+). The dynamics observed in the first fragment (A-C2H2+) is not discussed in this article.
Fig. (click to enlarge)  

Original publication: Nature Communications 6, DOI:10.1038/ncomms8909
XUV excitation followed by ultrafast non-adiabatic relaxation in PAH molecules as a femto-astrochemistry experiment

A. Marciniak, V. Despré, T. Barillot, A. Rouzée, M.C.E. Galbraith, J. Klei, C.-H. Yang, C.T.L. Smeenk, V. Loriot, S. Nagaprasad Reddy, A.G.G.M. Tielens, S. Mahapatra, A. I. Kuleff, M.J.J. Vrakking & F. Lépine

Contact

Prof. M. Vrakking
Tel. 030 6392 1200

 
     
 


How long does it take an electron to tunnel?

25 May 2015

The combination of ab-initio numerical experiments and theory shows that optical tunnelling of an electron from an atom can occur instantaneously.

How long does it take an atom to absorb a photon and loose an electron? And what if not one but many photons are needed for ionization? How much time would absorption of many photons take? These questions lie at the core of attosecond spectroscopy, which aims to resolve electronic motion at its natural time scale.

Ionization in strong infrared fields is often viewed as electron tunnelling through a potential barrier, created by the combination of the atomic potential that binds the electron and the electric field of the laser pulse that pulls the electron away. Thus, unexpectedly, attosecond spectroscopy finds itself facing an almost age-old and controversial question: how long does it take an electron to tunnel through a barrier?

In the paper by Torlina et al, this question is studied by using the so-called atto-clock setup. The attoclock uses the rotating electric field of a circularly polarized laser pulse as a hand of the clock. One full revolution of this hand takes one laser cycle, about 2.6 fs for experiments with 800 nm pulse of a Ti-sapph laser. As the electric field rotates, so does the tunnelling barrier. Thus, electrons tunnelling at different times will tunnel in different directions. This link between time and direction of electron motion is what allows the attoclock to measure times. In every clock, a time zero must be established. In the attoclock, this is done by using a very short laser pulse, which lasts only one-two cycles. Tunnelling occurs in a small window where the rotating electric field passes through its maximum.

Next, like any other clock, the attoclock must be calibrated. One has to know how the time of electron emission - its exit from the tunnelling barrier - maps onto the angle at which the electron is detected. This calibration of the attoclock has now been accomplished by Torlina et al, with no ad-hoc assumptions about the nature of the ionization process or the underlying physical picture.

Combining analytical theory with accurate numerical experiments, and having calibrated the attoclock, the authors could finally carefully look at delays in electron tunnelling. They arrive to the surprising answer: this time delay may be equal to zero. At least within the realm of non-relativistic quantum mechanics, the electron tunnelling out of the ground state of a Hydrogen atom spends zero time under the tunnelling barrier. The situation may change, however, if this electron encounters other electrons on the way, which may become important in other atoms or molecules. The interaction between the electrons may lead to delays.

Thus, the attoclock provides a unique window not only into the tunnelling dynamics, but also into the interplay of different electrons that participate in the ionization process, and how the electrons staying behind readjust to the loss of their comrade.

Original publication: Nature Physics

Lisa Torlina, Felipe Morales, Jivesh Kaushal, Igor Ivanov, Anatoli Kheifets, Alejandro Zielinski, Armin Scrinzi, Harm Geert Muller, Suren Sukiasyan, Misha Ivanov, Olga Smirnova, Nature Physics 11, 503-508 (2015) (doi:10.1038/NPHYS3340)

https://Doi:10.1038/nphys3340

Contact

Dr. Olga Smirnova

Prof. M. Ivanov


<graphik follows>

Fig. 1:
Ionization times (left axis) reconstructed using the ARM theory from offset angles (right axis) obtained numerically using TDSE calculations. Red circles are the numerically calculated offset angles, divided by the laser frequency, θ/ω. Blue diamonds show the offset angles with the correction due to the substraction of the pulse envelope effect, ti0=θ/ω-|Δtienv(θ,ppeak)| . Green inverted triangles show the Coulomb correction to the ionization time evaluated at the peak of the photoelectron distribution, |ΔtiC(θ,ppeak|. Orange triangles show the ionization times we obtain by applying the reconstruction procedure defined by equation (4) in the paper. In terms of the figure, this is simply the result of subtracting the green curve from the blue curve

Abb. 1 (click to enlarge)  
 
     
 


European researchers light the way towards top-level laser science and innovations

05. August 2015

LASERLAB-EUROPE, the consortium of major European laser research organisations, enters a new phase of collaboration from 2015 until 2019. In a very competitive call the consortium has been successful in securing EC funding of 10 million euros in Horizon 2020.

OPCA system
High repetition rate Optical Parametric Chirped Pulse Amplifier (OPCPA) system at MBI. Photo: MBI

Lasers are important tools in modern technologies, medical science and research. Recently the field of advanced lasers has experienced remarkable breakthroughs in laser technologies and novel applications. Laser technology is a key innovation driver for highly varied applications and products in many areas of modern society, thereby substantially contributing to economic growth.

For example, an ageing population brings with it an increased demand on healthcare systems. Screening and medical imaging methods based on photonics will strengthen preventive medicine and the early detection of diseases. In this context, LASERLAB-EUROPE will realise new techniques and tools for advanced microscopy and biomedical devices, develop novel medical therapies and biosensors as well as use the emerging relevant applications of laser-driven particle beams in radiotherapy.

'The participating laboratories cover advanced laser science and applications in most domains of research and technology, with particular emphasis on areas with high industrial and social impact, such as bio- and nanophotonics, materials analysis, biology and medicine. Through our strategic approach, LASERLAB-EUROPE will strengthen Europe's leading position and competitiveness in these key areas', says Prof. Claes-Göran Wahlström of the Lund Laser Centre in Sweden, who coordinates the LASERLAB-EUROPE consortium.

In the upcoming 4-year phase, starting in December 2015, LASERLAB-EUROPE will comprise 33 of the leading European laser infrastructures and, together with subcontractors and associate partners, involve coordinated activities in 21 countries. The members offer free access to key complementary laser facilities in Europe with performances at the international forefront of laser technology, including two Free Electron Lasers (FELs). This allows guest scientists from academia as well as from industry to carry out leading-edge research for the advancement of knowledge in a wide range of scientific domains, thus serving a very broad and interdisciplinary community.

Within LASERLAB-EUROPE, the Max Born Institute collaborates in several joint research activities, provides access to its application laboratories for guest scientists, and is responsible for the administrative project management.

Contact
Prof. Claes-Göran Wahlström
Lund Laser Centre, Lunds Universitet
P. O. Box 118, SE-221 00 Lund, Sweden
www.laserlab-europe.eu

Information may be obtained also from:
Daniela Stozno, MBI, Tel.: +49/30/6392-1508

 
     
 


Probing molecular chirality on a sub-femtosecond timescale

29 June 2015

New nonlinear all-optical method of detecting chiral molecules is proposed and demonstrated. It is a lot more sensitive than standard all-optical techniques and also allows for sub-femtosecond temporal resolution of chiral dynamics.

Two enantiomers of a chiral molecule are just like our left and right hands: they are mirror images of each other. They have identical physical properties unless they interact with chiral light. However, in linear chiroptical spectroscopies the chiral response, which is often called the "chiral dichroism", is very small, at the level of 10-4 - 10-5 of normal linear optical response such as light absorption. This creates major challenges for time-resolved measurements. These challenges are met in the paper by Cireasa et al, where a new approach to chiroptical detection is demonstrated and analyzed.

The new approach is based on high harmonic spectroscopy. High harmonic generation occurs when an intense femtosecond laser pulse is focused in a gas. It can be understood as a sequence of three steps: ionization in a strong infrared (IR) field, laser-induced acceleration of the liberated electron, and its recombination with the parent ion, all within one laser cycle. Recombination results in emission of coherent radiation extending from the vacuum ultraviolet to the soft X-ray region.

R. Cireasa et al looked at how the chiral structure of the molecule affects this process. While the liberated electron is driven by the laser field, the same is happening to the hole. What's more, the laser-driven hole motion is chiral and enantio-sensitive, thanks to the chiral structure of the molecule. When the returning electron recombines with the hole, the enantio-sensitive nature of the hole motion makes the emitted light enantio-sensitive. As a result, very small ellipticity of the driving laser field, at the level of about 1%, is sufficient to distinguish between the harmonics emitted by left-handed or right-handed molecules, with signals differing by 2-3%.

High harmonic generation can be viewed as pump-probe spectroscopy. Ionization acts as a pump, starting the electron-hole dynamics. Recombination acts as a probe, which maps the electron-hole dynamics on the emitted light. The pump-probe delay is controlled by the oscillation of the laser field, which drives the electron. The energy of the returning electron depends on how much time it has spent in the field. As a result, harmonics with different energies are emitted at different times, providing the mapping between the harmonic number and the pump-probe delay. In a nutshell, the harmonic emission records a movie of the recombining system, with each harmonic representing a single frame. Huge bandwidth of the harmonic spectrum leads to very high temporal resolution, about 0.1 fsec or better. R. Cireasa et al have used this property to reconstruct the chiral component of the hole dynamics from the experimentally measured chiral dichroism, with 0.1 fsec resolution.

OlgaProbing Fig.: Calculations of the ellipticity dependence of the high-harmonic signal in S-epoxypropane (a) and R-epoxypropane (b) confirm that the chiral-sensitive signal is particularly strong around harmonics 41 - 43, where the main (chiral-insensitive) high-harmonic channels XX and AA interfere destructively. For each harmonic the signal is normalized to its maximum as a function of ellipticity.
Fig. 1 (click to enlarge)  

Original publication: Nature Physics, Vol. 11, 654-658, June 2015
Probing molecular chirality on a sub-femtosecond timescale

dx.doi.org/10.1038/nphys3369

Full citation
R Cireasa, AE Boguslavskiy, B Pons, MCH Wong, D Descamps, S Petit, H Ruf, N Thiré, A Ferré, J Suarez, J Higuet, BE Schmidt, AF Alharbi, F Légaré, V Blanchet, B Fabre, S Patchkovskii, O Smirnova, Y Mairesse, VR Bhardwaj

Contact

Dr. Olga Smirnova
S. Patchkovskii

 
     
 


Are rogue waves predictable?


28 May 2015

A comparative analysis of rogue waves in different physical systems comes to the surprising conclusion that these rare events are not completely unpredictable.

 

Monsterwelle
Detail of "The Great Wave off Kanagawa" by Katsushika Hokusai, which has been frequently discussed to depict an ocean rogue wave.
Metereological events often prove to be rather unpredictable, i.e., the "storm of the century" may well prove to be surpassed by yet another storm just in the subsequent year. From an insurance point of view, resulting damage often proves to be be well beyond any statistical prediction. Such phenomena generally underlie extreme value statistics, featuring a prevalent appearance of extreme events and contrasting long-term observations of rather normal events in the respective system. Rogue waves, also known as freak waves, are yet another example for such dynamics. While being extremely rare events, their appearance may cause considerable damage to the hull of ships.

 

The precise origin of rogue waves is still disputed. Moreover, it is unclear whether rogue waves can be predicted. Maybe, it is possible to issue a last-instant warning from observations of recorded wave heights? Do characteristic patterns exist that herald the impact of such a rogue wave? Unfortunately, there are only a few recordings of such ocean freak waves. Consequently, it may well take many more decades to answer those questions based on oceanic observations only. Nevertheless equivalent physical systems exist, which allow an exploration of this aspect at a substantially more solid statistical basis.

This is the point where the work of Simon Birkholz and coworkers sets in. Based on a statistical analysis of data in three different physical systems, the group conducted a detailed analysis on the predictability and determinism in the respective system. This analysis included original data of the famous New Year’s Wave, which hit the Draupner platform on January 1, 1995 as well as results of the Jalali group at the University of California at Los Angeles (UCLA), and finally data in a multifilament scenario measured at the Max-Born-Institut in Berlin. ┬áIn the multifilament system, one can directly observe the rogue waves as short light flashes in the intensity profile. The wave height of the ocean system corresponds to light intensity in the optical systems.

The surprising result of this comparative analysis is that rogue events appear to be very much predictable in certain system, yet are completely stochastic and therefore unpredictable in others. In other words, rogue wave statistics does not enable any conclusion on predictability and determinism in the system. It is simply not true that rogue events per se appear out of nowhere and disappear without a trace. Ocean waves play a particular role here. Other than previously assumed, they are not completely stochastic. Therefore it is not true that they “appear out of nowhere and leave without a trace”, which has often been claimed to be a characteristic feature of ocean rogue waves. Nevertheless, practical predictions are still far away and may only enable a last-second warning of these “monsters of the deep”.

d Fig.: Snapshot of a rogue event in multifilament dynamics recorded in
a xenon cell at 60 times the critical power for filamentation.
The optical fluence is plotted as a function of position on the optical detector.
Interview

only in German available

Logo - Das Wissenschaftsmagazin: Monsterwellen und ihre Vorhersehbarkeit von: Michael Kurth
gesendet am 19.06.2015 auf Norddeutschen Rundfunk, NDR Info (Ausschnitt Dauer: Minute ca. 7:33 bis 14:07)

   

Original publication: Physical Review Letters 114, 213901 Predictability of Rogue Events

Simon Birkholz, Carsten Brée, Ayhan Demircan, and Günter Steinmeyer (Editor’s suggestion)

Nature Photonics, Vol. 9, September 15: http://www.nature.com/nphoton/journal/v9/n9/pdf/nphoton.2015.161.pdf

Das Physikportal: Pro-Physik.de: http://www.pro-physik.de/view/0/login2.html

Contact

S. Birkholz
Dr. G. Steinmeyer

 
     
 


Rubicon fellowship for Geert Reitsma

10 April 2015

The Netherlands Organisation for Scientific Research (NWO) rewards MBI researcher Geert Reitsma a prestigious Rubicon Grant for his proposal "Filming biomolecules in action". With this grant he will continue his career at the MBI.

Many processes of life rely on ultrafast movements of complex molecules. One example for an ultrafast movement happens right now while you are reading these lines: When retinal in our eye absorbs light, the molecular structure changes extremely fast. This change ultimately induces an electrical signal transduced to the brain with the result that we see the light. The general sequence of this process is very well understood. However, the underlying electronic and nuclear rearrangement of the molecule is not adequately understood. The ideal way to get this understanding is filming such a process. As the process is very fast, the frame rate needs to be very high to capture individual rearrangements. One prerequisite to obtain this frame rate is very advanced ultrafast laser technology. Here, at the MBI, Reitsma will have access to the most advanced laser systems allowing him to produce these movies of biomolecules in action.

Dr. Reitsma did his PhD research in the Quantum Interactions and Structural Dynamics group at the Zernike Institute for Advanced Materials in Groningen (NL). He received his PhD from the University of Groningen on Dec. 1, 2014. The Netherlands Organisation for Scientific Research (NWO) is the most important science-funding body in the Netherlands and aims to ensure quality and innovation in science. Facilitated by NWO's Rubicon programme, 60 PhD graduates per year will be conducting research at top foreign institutes. Through Rubicon, NWO gives talented young scientists the opportunity to gain international research experience as a stepping stone to a scientific career.

More information: NWO's Rubicon programme

http://www.nwo.nl/en/news-and-events/news/2015/

Contact

Dr. Geert Reitsma

 

 
     
 


Fast Gold

26. March 2015

A new laser plasma acceleration scheme for heavy ions is found, leading to a significant boost of their kinetic energy by Coulomb explosion.

We are all made from stardust - this phrase contains a lot of unknown but exciting physics, which the poem writer probably did not want to tell. Within this context the top ten ranking of open problems in physics lists the question related to the synthesis of the heavy elements - stardust. To gain a deeper inside, from an experimental point of view, heavy ions with very high velocities are needed. The collision of heavy particles at high kinetic energy enables the study of the resulting reaction products. Beyond basic research, beams or pulsed beams of heavy ions are in the focus of applications in solid state physics as well as in bio-medical areas.

Such beams are produced with particle accelerators which belong to the biggest and most complex machines for research today. In order to make things more compact there is a strong motivation for searching new concepts and technologies in particle acceleration. One approach is acceleration by plasma created by laser pulses at relativistic intensities. The term relativistic indicates here that the intense light field causes an electron motion close to the speed of light and hence relativistic effects determine the properties of the laser-plasma interaction. Enormous fields with a strength of the order of megavolts per micrometer can be created in a well-defined direction leading to fast (some ten percent of light velocity) light and heavy ions. Due to the extreme acceleration these ion beams have some striking characteristics which are studied and applied in experiments.

The challenge of heavy ion acceleration results directly from basic principles: Ions are accelerated proportional to their charge to mass ratio Z/A leading to higher velocity for lighter elements. Laser plasma acceleration of really heavy ions e.g. gold, had been quite inefficient for this reason as very high ionization degrees are difficult to obtain. This limitation is overcome in our experiments with freestanding gold foils of nanometer thickness: which provide an so far unexpected high degree of ionization and specific distribution of the heavy material (Z>40+ for Gold ions). Compared to former experiments we achieved kinetic energies of the gold ions with 1 MeV per nucleon with an order of magnitude less laser energy.

Common laser plasma driven acceleration models assume an averaged degree of ionization, which follows with a fixed, spatially uniform electron density. From our theoretical considerations and simulations we recognized, that the obtained high kinetic energies of the heavy ions are not consistent with an average ionization. Instead, we found for the ultra-thin targets a spatially varying degree of the ionization, with the highest degree of ionization at the target boundaries (found by simulation - cf. Fig.) - where it greatly enhances the electrical field and thus the force driving the ion acceleration.

Extrapolation of our results envisions the study of nuclear processes if heavy ions with adequate kinetic energies will be produced by using femtosecond lasers with 100 Joule pulse energy.

Original publication: Physical Review Letters

Full citation:

J. Braenzel, A.A. Andreev, K. Platonov, M. Klingsporn, L. Ehrentraut, W. Sandner, M. Schnuerer, "Coulomb-Driven Energy Boost of Heavy Ions for Laser-Plasma Acceleration", Physical Review Letters 114, 124801 (2015)

doi: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.124801

Contact

Julia Braenzel

Dr. Matthias Schnuerer


Fig. 1:
Laser (1.3J @ 35 fs) irradiated 14 nm thick gold foils. The picture shows dependence of maximal ion energy on its ionization energy - as the experiment delivered (pink squares). We found good agreement with the 2D -PIC simulation - blue squares and with the prediction of our new theoretical model -blue line in contrast to the established, old model - black line

 

Fig. 1 (click to enlarge)  
 
     
 


The exciting story of He atoms in strong laser fields

24 March 2015

Astonishingly, the interaction of atoms with intense laser radiation does not necessarily lead to ionization but also to bound excited states. As investigated in a recent publication, H. Zimmermann and coworkers have experimentally confirmed predictions of a model that provides an intuitive understanding of why and how strong-field excitation happens.

When exposing atoms to modern high intensity laser radiation it seems as if it is more appropriate to consider light as a classical electromagnetic wave rather than as a stream of particles called photons. The photon picture was revolutionarily put forward by Einstein to explain light-matter interaction at low light levels, where an atom picks a single photon of appropriate energy to get either excited or ionized (the famous photo effect), see Figure 1a. The enormous number of photons compressed in a short laser pulse translates into an enormous electric field strength of the electromagnetic wave of almost 1 billion V/cm compared to 1 V/cm for light sources available back in Einstein's time. These field strengths perturb the atom profoundly enforcing most likely ionization.

The fact that excitation can still be observed in such strong laser fields and conveniently explained in the wave picture has been demonstrated in the recent publication in Physical Review Letters by H. Zimmermann and coworkers. They exposed He atoms to an intense laser pulse with a duration of 40 femtoseconds and measured the final excited state distribution. (To visualize the time scale: the ratio of the laser pulse duration to one second is the same as the ratio of the duration of a work day to the age of the universe). Although the electron is heavily shaken during the laser pulse, the He atom manages to prevent ionization and gets excited. Most importantly, the authors favorably compare their results with the so-called "frustrated tunneling ionization" (FTI) model, established earlier by Nubbemeyer et al. from MBI, see Figure 2. The FTI model is based on the strong-field tunneling picture put forward by Russian physicist L. Keldysh fifty years ago. Tunneling of the electron occurs instantaneously whenever the strong laser field makes the otherwise insurmountable binding potential penetrable and, according to Keldysh's theory, the electron disappears into thin air. The tunneling process itself, however, does not inevitably lead to the liberation of the electron. Tunneling ionization can be frustrated so that the electron remains bound in an excited state if one considers the subsequent dynamics of the electron in the combined laser and Coulomb field of the ionic core properly. At certain circumstances the electrons do not gain enough "escape" energy neither from the laser nor through collisions after the tunneling process to leave the attractive potential of the ionic core, see Figure 1b).

With the experimental investigation, Zimmermann et al. corroborate the frustrated tunneling model as an intuitive way to comprehend excitation of atoms in the strong field tunneling regime. The results clearly elucidate the fundamental process of atomic excitation at highest laser intensities as a process, which combines the quantum mechanical process of tunneling with the classical description of an electron in an electromagnetic wave plus a Coulomb field.

Original Publication: Physical Review Letters: doi.org/10.1103/PhysRevLett.114.123003

Full citation:
H. Zimmermann, J. Buller, S. Eilzer, and U. Eichmann, "Strong-Field Excitation of Helium: Bound State Distribution and Spin Effects", Physical Review Letters, 114, 10.1103 (2015)

contact

Prof. Dr. Ulrich Eichmann

BildfolgtEich1a

Fig. 1a): Einsteins view of excitation at low light levels: A photon (green dot) hits an atom in its ground state and instantaneosly promotes the electron to a higher orbit only, if the photon energy matches the transition energy.

 

Fig. 1a (click to enlarge)  
BildfolgtEich1b

Fig. 1b): The strong laser pulse interacts with a ground state electron heavily influencing its trajectroy and its final relaxation in a higher orbit, although the photon energy does not match the transition frequency.

Fig. 1b (click to enlarge)  
BildfolgtEichFig2 Fig. 2: Comparison of the distribution of principal quantum number n: Measured n-distributions for a laser intensity of 1,8x1015 W/cm2 (blue dots) and 2,9x1015 W/cm2 (red squares).Calculated n-distribution according to the semiclassical FTI model at field strengths of 1015W/cm2 (open squares) and 1,4x1015 W/cm2 (open diamonds).
Fig. 2 (click to enlarge)  
 
     
 


Classical or not? Physics of nanoplasmas


24. March 2015

The interaction of an intense laser pulse with a nanometer-scale particle results in the generation of an expanding nanoplasma. In the past, nanoplasma dynamics were typically described by classical phenomena, like the thermal emission of electrons. In contrast, a new study on the interaction of intense near-infrared (NIR) laser pulses with molecular oxygen clusters now demonstrates that phenomena, which can only be described quantummechanically, play an important role. For the first time, evidence of efficient formation of autoionizing states in nanoplasmas is found. Autoionization of so called superexcited states of atomic oxygen is directly observed on a nanosecond time scale, whereas indirect signatures are visible for decay processes occurring on shorter time scales. Autoionization is found to take place in various systems and is expected to be important also in the interaction of finite systems with intense extreme-ultraviolet (XUV) and X-ray pulses from novel free-electron laser sources.

Following the interaction of intense NIR laser pulses with clusters, the recorded electron spectra typically show a smooth distribution. In the past, the absence of discrete state signatures in these spectra led to the conclusion that the dynamics of charged particles during the cluster expansion can be well described by fully classical behavior. As a consequence, simulations that model the interaction of intense lasers with clusters, nanoparticles or large molecules, often make use of quasiclassical approaches. With the advent of novel laser sources and time-resolved techniques during the last year, this picture began to falter. Recently, extensive formation of excited atoms in nanoplasmas driven by electron-ion recombination processes was reported. When an atom with 2 electrons in excited states is formed, it may decay via an electron correlation effect, where one electron is released into the continuum, while the second electron relaxes to a lower bound state. However, since the electrons emitted via such autoionization processes exchange kinetic energy with the cluster environment, they had not been observed in experiments so far.

In a collaboration led by scientists from the Max-Born-Institut, the first evidence of autoionization following intense NIR laser-cluster interactions is now reported. In the current issues of Physical Review Letters [114, 123002 (2015)] Bernd Sch├╝tte, Marc Vrakking and Arnaud Rouz├ęe, and their colleagues Jan Lahl, Tim Oelze and Maria Krikunova from the TU Berlin present results obtained from oxygen clusters. This system was chosen, because oxygen atoms have previously been shown to exhibit long-lived autoionizing states. In the present study, clear peaks were observed in the electron spectrum from oxygen clusters ionized by intense NIR pulses (Fig. 1). These peaks could be assigned to well-known autoionizing states, and it was shown that they decay on a nanosecond time scale, when the cluster has already significantly expanded. Therefore, the influence of the environment on the electrons emitted via autoionization was negligible. The observed autoionization contributions were found to be very sensitive on the intensity of the NIR laser pulse. At higher intensities, the autoionization peaks were blurred out, but still visible. These results indicate that autoionization plays an important role in many experiments that study the interaction of intense laser pulses with nanometer-scale systems, even when these processes cannot be directly observed in the electron spectrum. Previously, it was demonstrated that the observed nanoplasma dynamics following intense XUV and NIR ionization of clusters are similar, and therefore, the current results are expected to be highly relevant as well for experiments at novel free-electron lasers. The experimental findings of autoionization are also important for improving theoretical models of nanoplasmas in the future in order to gain a better understanding of the underlying microscopic processes.

The presented results demonstrate that a description of nanoplasma dynamics by classical approaches is insufficient. Quantum phenomena like autoionization play an important role during the expansion of clusters following the interaction with intense light pulses.

Originalpublication: Physical Review Letters

Full citation:

Bernd Schütte, Jan Lahl, Tim Oelze, Maria Krikunova, Marc J. J. Vrakking and Arnaud Rouzée, "Efficient autoionization following intense laser-cluster interactions", Physical Review Letters 114, 123002 (2015)

doi: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.123002

Contact:

Dr. Bernd Schütte

Prof. Marc J. J. Vrakking

Dr. Arnaud Rouzée

schuette2015

Fig. 1: (a) Two-dimensional electron momentum map emitted from O2 molecules, showing an anisotropic distribution of electrons peaked along (vertical)  the NIR laser polarization. (b) In the corresponding kinetic energy spectrum, the observed peaks are attributed to above-threshold ionization and Freeman resonances. (c) The electron momentum map from O2 clusters with an average size of 2400 molecules exhibits a much more isotropic behavior. (d) In the kinetic energy spectrum, three clear peaks emerge that are assigned to autoionization of superexcited atomic states.

 

Fig. 1  
 
     
 


New Insights into the photophysics of the DNA base thymine


11th February 2015

DNA stores our genetic code. Solar UV radiation has sufficiently high energy to basically break bonds of the DNA and thus cause DNA damage. Although DNA (e. g. in our skin cells) is exposed to intense UV light irradiation, DNA proves to be surprising photostable. It is well established that this is due to efficient mechanisms that convert electronic energy into other forms of energy, in particular heat. An important role is played by so-called conical intersections between electronic excited potential energy surfaces and the ground state potential energy surface. These conical intersections are associated with structural changes of the molecules. The exact pathways back into the electronic ground state however are topic of intense research.

Although DNA is a macro molecule with billions of atoms (in case of human DNA), it can still be divided into only a few structural (and functional) elements: four DNA bases, a sugar moiety and a phosphate group. The absorption of UV light exclusively takes place in the DNA bases. For this reason it is a common scientific approach to investigate the UV response of DNA bases, first.

A team of scientists from MBI and universities of Hokkaido and Hirosaki in Japan have for the first time investigated the DNA base thymine in aqueous solution by time-resolved photoelectron spectroscopy and questioned existing ideas about the excited-state relaxation process in this base. So far was supposed that a significant fraction of the excited-state population remains in a dark nπ* state instead of immediately returning to the ground state via a conical intersection. This dark state cannot be observed by optical spectroscopy (e. g. transient absorption or fluorescence upconversion), directly. Corresponding limitations however do not exist for photoelectron spectroscopy.

By combining experiment and simulation, for the first time two different relaxation pathways were identified. Both pathes evolve in the first excited state (ππ*). The faster reaction path is associated with a twist of the aromatic ring and leads to repopulation of the electronic ground state within 100 fs. The second path involves an out-of-plane motion of the carbonyl group, and the molecule returns to the ground state within 400 fs. The scientists did not find any indication for an important role of the second excited nπ* state and conclude that this state is not involved in the relaxation process.

Original publication:
Franziska Buchner, Akira Nakayama, Shohei Yamazaki, Hans-Hermann Ritze, Andrea Lübcke
Excited-State relaxation of hydrated thymine and thymidine measured by liquid-jet photoelectron spectroscopy: experiment and simulation, JACS,
JACS, DOI: 10.1021/ja511108u

folgt in K├╝rze

Fig. 1: After UV excitation thymine evolves on the (ππ*) excited state surface along two different reaction coordinates. The first involves a twisting of the aromatic ring, the second an out-of-plane motion of the carbonyl group. In contrast to existing ideas, the nπ* state does not seem to be involved in the relaxation process..

Fig. 1 (click to enlarge)


Contact:
Dr. Andrea Lübcke Tel: 030 6392 1207



 
     
 

Nonlinear resonance disaster in the light of ultrashort pulses


10th February 2015

Ultrashort light pulses from modern lasers enable temporal resolution of even the fastest processes in molecules or solid-state materials. For example, chemical reactions can, in principle, be traced down to the 10-fs time scale (1 femtosecond (fs) = 10-15 s). Ten femtoseconds correspond to a few oscillation cycles of the light field itself. Nevertheless, there is a class of optical processes that does not exhibit any measurable delay relative to the ultrafast light oscillation and which has been termed “instantaneous”. This class of processes includes nonlinear optical harmonic generation at multiple frequencies of the input field. This process is commonly used to generate the green light of laser pointers from invisible infrared light. These processes are normally used far away from a resonance to avoid losses.

In a collaborative effort, researchers of the Max-Born-Institut, the Weierstraß-Institut as well as the Leibniz-Universität Hannover now experimentally demonstrated for the first time that conditions exist where optical harmonic generation becomes non-instantaneous. Analyzing third-harmonic generation in titanium dioxide thin films, a lifetime of 8 fs was found, i.e., non-instantaneous behavior. Nevertheless, this process still qualifies as one of the fastest processes ever resolved with femtosecond spectroscopy.

Detailed theoretical modeling of these surprising findings indicates that this non-instantaneous response may only occur if there is a resonance of the third harmonic in the optical material. In turn, the generated material response persists to oscillate several cycles after the excitation has already ceased. Concomitantly, third-harmonic radiation is emitted. The process therefore appears like an atomic “resonance disaster”. Similar to mechanical oscillators, this atomic system therefore shows a non-instantaneous behavior.

These findings have important consequences for femtosecond measurement techniques and possibly also for ultrashort-pulse generation. These methods have always relied on an instantaneous nature of harmonic generation and related effects. Similar to soldiers who avoid marching in step on a suspension bridge, one therefore also has to carefully avoid optical resonances when measuring extremely short laser pulses.

Original publication:
Michael Hofmann, Janne Hyyti, Simon Birkholz, Martin Bock, Susanta K. Das, Rüdiger Grunwald, Mathias Hoffmann, Tamas Nagy, Ayhan Demircan, Marco Jupé, Detlev Ristau, Uwe Morgner, Carsten Brée, Michael Woerner, Thomas Elsaesser, Guenter Steinmeyer
Noninstantaneous polarization dynamics in dielectric media
OPTICA doi.org/10.1364/OPTICA.2.000151

 

ResponseJPG

Figure 1: Reaction of SiO2 and TiO2 to a short pulsed light field. In SiO2 the displacement of electron shell follows the exciting electric field. Immediately after the end of the pulse, this oscillation ceases, too. In contrast, in TiO2, an oscillation build-up is observed at the third harmonic of the exciting field. This oscillation continues beyond the end of the pulse. Insets show pictures of crystalline modifications for both optical materials (Photographs by Didier Descouens, CC BY 3.0 and Rob Lavinsky, CC-BY-SA-3.0).

Fig. 1 (click to enlarge)

visualize4

Movie: Reaction of SiO2 and TiO2 to a short pulsed light field. The electric field is visualized by the central arrow. The resulting displacement of the electron shell is shown in a simple atomic picture for both materials. Third-harmonic emission is indicated by a blue color of the shell. In SiO2, both the resulting oscillation as well as the harmonic emission immediately cease after the end of the exciting pulse. In contrast, TiO2 exhibits a resonant build-up of the third-harmonic oscillation, which persists beyond the duration of the exciting pulse.

Fig. 2 (click for animation - AVI-file)  


Contact
Dr. Günter Steinmeyer Tel: 030 6392 1440

 
     



Earlier Highlights and News can be found in the Archive. ... here